1. Field of the Art
It is well known that Mo-Bi composite oxide catalysts are useful for selective reactions such as vapor phase catalytic oxidation reactions for producing acrolein from propylene or methacrolein from isobutene or tertiary-butanol, a vapor phase catalytic ammoxydation reaction for producing acrylonitrile from propylene or methacrylonitrile from isobutene, and a vapor phase catalytic oxidative dehydrogenation reaction for producing butadiene from butene. It is also well known that these catalysts have widely been put to practical use on an industrial scale.
2. Prior Art
Many patent publications with reference to the compositions of Mo-Bi oxide catalysts in these various reactions and the processes for producing them are also well known. A part of such publications are: Specifications of Japanese patent examined publication Nos. 3670/64, 4763/73, 3498/74, 17253/73, 1645/73, 41232/80, 14659/81, 23969/81, 52013/81 and 26245/82; and those of Japanese patent unexamined publication Nos. 503/73, 514/73, 52713/73, 57916/73, 54027/73, 76541/84, 47144/80, 20610/80, 84541/80 and 122041/85.
All of the aforementioned patent publications relate to Mo-Bi composite oxide catalysts. However, all but Japanese patent unexamined publication Nos. 47144/80 and 76541/84, which disclose the preliminary production of a Mo-Bi or W-Bi composite in the course of producing the composite oxide catalysts, use bismuth nitrate as a raw material of Bi in Examples, and in fact, a water soluble bismuth compound, viz. bismuth nitrate or a hydroxide thereof is recommended also in their descriptions.
It can be readily understood that when elements as the components of a composite oxide catalyst are incorporated into a composite by the use of a system including water as a dispersing medium, use of water soluble compounds as the compounds is the most common sense measure which one skilled in the art would conceive as a method for uniform dispersion in a catalyst. In fact, we have also confirmed that uniform dispersion of the compounds as aqueous solutions using such water soluble raw materials are effective in the case of the elements used for these composite oxides such as iron, cobalt, and nickel.
However, in the case of this composite oxide catalyst, it is not appropriate to use bismuth nitrate in a region having a high Bi content. The reason is that under conditions where bismuth nitrate is used, catalysts in the region having less Bi content are superior in their performance.
In Mo-Bi composite oxide catalysts, high Bi content is an important factor from the viewpoint of the catalyst life, and thus the above mentioned problem is serious.